For more information on image file backgrounds, see Image file backgrounds.
To create an animated image file background
This automatically enters an expression (=frame) into the Image Number attribute. The Image Number is equal to the Timeline’s current frame.
If a non-standard animated image sequence logic is desired, you can either edit the expression on the Image Number attribute (right-click Image Number and select Edit Expression) and enter the desired changes. For example, if you wanted to slow down the image sequence to "playback" at half speed you could add a divide by 2 to the expression.
Alternately, you can remove the expression from the Image Number attribute and set key frames on the Image Number attribute just like animating any other attribute. This allows for complex image sequence logic, for example reversing the image sequence or holding the first frame, then "playing" then holding the last frame.